The present invention relates to a high-purity, opaque quartz glass having an excellent capability of intercepting heat rays, and a method for producing such a high-purity, opaque quartz glass. Further, it relates to a flange and a tube made of such a high-purity, opaque quartz glass, and methods for producing them.
Because of high capability of intercepting heat rays, an opaque quartz glass has been widely used hitherto for furnace tubes and flanges for use in the production of semiconductors, etc. Since heat rays reflect on a surface of each cell contained in the opaque quartz glass, the dimension of the cell contained in the opaque quartz glass has a large influence on the capability of intercepting heat rays.
Most opaque quartz glass presently used contains closed cells having an average size of more than 50 .mu.m in a proportion of 1.times.10.sup.6 or less per 1 cm.sup.3, more specifically 1.times.10.sup.4 -1.times.10.sup.5 per 1 cm.sup.3, many of the closed cells having sizes of 100 .mu.m or more. Some opaque quartz glass contains closed cells having an average size of 50 .mu.m or less, but the number of the closed cells is 1.times.10.sup.6 or less per 1 cm.sup.3. Therefore, the whole surface area of the closed cells is inevitably as small as less than 50 cm.sup.2 per 1 cm.sup.3 in such opaque quartz glass resulting in a limited capability of intercepting heat rays.
The capability of intercepting heat rays has a close relation to transmittance of light, particularly for near infrared rays. Generally, the lower the transmittance for near infrared rays, the higher the capability of intercepting heat rays. Presently used opaque quartz glass has 10-50% of transmittance for near infrared rays (.lambda.=900 nm) at a thickness of 3 mm, and presumably 30-60% at a thickness of 1 mm. These transmittance levels are too high to sufficiently intercept heat rays.
A opaque quartz glass is conventionally produced by melting natural materials such as silica and quartz in an electric furnace, or by melting the natural materials together with blowing agents such as CaCO.sub.3. Since the opaque quartz glass thus produced contains impurities such as alkali substances, it is difficult to use the resulting glass as materials for parts for use in the production of semiconductors, etc.
Conventional opaque quartz glass produced from natural materials and blowing agents contains over 1 ppm of impurities such as Na and Fe. In addition, since a conventional opaque quartz glass flange, for instance, is produced by forming a bulk of opaque quartz glass and machining the bulk into a predetermined shape, large percentages of materials are lost, leading to an extremely poor yield and thus to a high production cost. Further, since a sealing surface of the conventional flange is ground, it suffers from degradation, because cells exposed on the sealing surface by grinding are etched.
The opaque quartz glass is useful for a furnace tube in a silicon wafer-treating apparatus, etc. A typical example of such a silicon wafer-treating apparatus is shown in FIG. 1. The apparatus comprises a heater 1, a furnace tube 2, silicon wafer 3, a support member 4 for supporting the silicon wafer 3, a heat-keeping cylindrical base member 5 for supporting the support member 4, and a base plate 6. A flange 21 is integrally mounted to the lower end of the furnace tube 2. A seal ring 7 is set between the flange 21 and the base plate 6.
Generally, the furnace tube 2 and the flange 21 are separately produced and connected together by welding, etc.
With respect to an opaque quartz glass tube, it is conventionally produced by pulling a molten opaque quartz glass obtained from natural materials such as silica and quartz in a tubular shape, or by centrifugally casting the molten opaque quartz glass in a rotary furnace. Since the conventional opaque quartz glass tube thus produced also contains large cells whose sizes have a broad particle distribution, it fails to sufficiently intercept heat rays. As a result, it is difficult to use such a glass tube as a heat-resistant part in the production of semiconductors, etc.
Further, since a force applied to a molten glass in the pipe-pulling process or the centrifugal casting process functions to make the cells long and narrow and to make the distribution of cells uneven, the resulting opaque quartz glass tube has poor mechanical strength.